This invention relates to continuous casting systems, and more particularly to a method and apparatus for controlling the molten liquid level in a continuous casting mold.
In a conventional continuous casting system, a plug or "dummy bar" is inserted through the bottom of an open-bottomed mold, and molten metal is then poured into the mold. The mold side walls are typically water-cooled, and the molten level cools significantly faster around the periphery of the mold. As the metal begins to solidify, a skin develops around its periphery and the plug is slowly withdrawn from the bottom of the mold. As the metal strand is withdrawn from the bottom of the mold, molten metal is continuously poured into the top of the mold, and a long strand of casting can be obtained from a relatively small mold.
In such continuous casting systems, it is important to maintain the level of molten liquid in the mold within some predetermined range. Obviously, it would be undesirable to have the mold become empty or to have the mold overflow, but level control on a much finer scale is also desirable. The quality of the cast steel can be enhanced by maintaining the molten liquid within the mold at some optimum level, and a variety of level control techniques have been used. There are two factors which can be controlled in order to maintain a desired liquid level within the mold, the rate at which the metal strand is withdrawn from the mold and the rate at which the liquid metal is poured into the mold. The withdrawal rate can be controlled by adjusting the casting speed, and the flow rate into the mold can be controlled, e.g., by adjusting a tundish throttle gate through which the liquid metal is poured. Different systems may control either one of these two factors, and some systems may use a combined control of both factors.
U.S. Pat. No. 2,832,110 issued to Carleton on Apr. 29, 1958, discloses a technique of manual operator control over the flow rate of liquid metal into the mold. Due to problems of operator error and fatigue, however, such manual control techniques have typically been replaced with automatic control. U.S. Pat. No. 4,306,610 issued to Ahmed on Dec. 22, 1981, U.S. Pat. No. 4,222,506 issued to Sakashita et al on Sept. 16, 1980, and U.S. Pat. No. 3,884,288 issued to Berry on May 20, 1975 all teach techniques of controlling the liquid level within the tundish, to thereby control the flow rate of molten liquid into the mold.
U.S. Pat. No. 4,019,562 issued to Shiraiwa et al on Apr. 26, 1977 discloses a technique for monitoring the rising rate of liquid level within the mold and controlling a tundish throttle gate valve to correspondingly control the rate of liquid flow into the mold.
U.S. Pat. No. 3,519,060 issued to Vischulis on July 7, 1970 discloses a technique in which the level of liquid within the mold is monitored to generate a control signal to an appropriate servo mechanism to control the rate at which liquid is poured into the mold. The Vischulis system utilizes a feedback-type servo loop for its control, and may exhibit an undesirably slow response time.
More recently, U.S. Pat. No. 4,226,278 issued to Osugi on Oct. 7, 1980 discloses a system wherein a plurality of thermocouple devices are used to detect the level of molten liquid within the mold, and a servo-mechanism including a microcomputer or the like incrementally opens or closes the tundish throttle gate in order to increase or decrease, respectively, the rate of liquid flow into the mold. The Osugi system merely detects when the mold level exceeds upper or lower limit values, with the control system being designed to fully open or fully close a valve momentarily to correct the level when it exceeds one of the limit values. The Osugi system is not apparently capable of the degree of highly accurate level control desired in current casting systems.
U.S. Pat. No. 3,300,820 issued to Tiskus and Wilson on Jan. 31, 1967 discloses a flow control system wherein liquid metal is poured directly from the ladle into the mold rather than through the intermediary of a tundish. In this system, mold level control is accomplished by monitoring the mold level and automatically manipulating a ladle stopper rod to control the rate at which the molten steel is poured into the mold. The throttle gate control signal in the '820 patent is generated by charging a capacitor with a signal proportional to the mold level error signal, so that larger mold level errors will result in faster charging of the capacitor. When the capacitor is charged to a predetermined level, a control pulse of constant duration is generated. The system will continue to generate these control pulses until the mold level error disappears.
The most significant drawback of this system is that, since each control pulse moves the throttle gate by a predetermined amount, the amount of adjustment to the throttle gate will depend upon the number of pulses generated. The speed with which an adjustment can be made will therefore depend upon the maximum frequency with which the pulses can be generated and also the maximum frequency with which the electromechanical throttle mechanism can be controlled. This results in an undesirable limitation on the responsiveness of the control system.
Recent studies have indicated that internal and surface imperfections in continuously-cast steel can be minimized by employing constant speed casting while also reducing the amplitude of mold level variations to approximately .+-.3 mm or less. In order to maintain such stringent mold level requirements, a more accurate mold level control system capable of quick response to minute level variations would be desired.